Small particles are normally prepared by pulverization. This method produces particles of irregular shapes and with a wide particle size distribution. The particles, even after milling, are normally big enough to scatter light as to render them visible to the naked eyes. In a number of cases, the disclosed procedure called for, after reaction and precipitation, further steps of drying, calcination and pulverization. In other cases, strict control of pH during the precipitation step was required.
EP 0 641 740 A1 discloses a process for the preparation of crystalline perovskite powders by precipitation. The procedure involves the introduction of an aqueous solution of mixed inorganic metal salts or organometallic compounds of a claimed group of elements and elements from a second group of claimed elements, into a stirred aqueous solution of inorganic or organic base preheated to 70-100.degree. C. at constant OH.sup.- concentrations. Thus, the use of a basic solution; and the careful control of OH.sup.- concentration is required.
WO 94/01361 discloses a method of making nanometer-sized particles, by continuously flowing a solution of a precursor in a solvent through a heated vessel and chemically reacting the solution to form particles, followed by flow into a cooled region for recovery of the particles. Quenching in a cooled region is an additional step requiring additional equipment.
U.S. Pat. No. 4,324,772; discloses a continuous process for producing hydroxyapatite from calcium oxide in water and phosphoric acid which comprises a two-stage reaction in which pHl control and reactant flow are carefully regulated. Control of the first stage pH between about 9.5 to about 11 and the second stage pH between 7.0 to 7.4 is required. Vigorous agitation is disclosed as critical.
U.S. Pat. No. 3,387,925 discloses a batch process of tricalcium phosphate from lime (calcium oxide) and phosphoric acid and found that small particle size may be produced by controlling the final pH, the reaction temperature and the rate of addition of the reactants. The disclosed temperature is between 20 to 50.degree. C. and the reactions were carried out in an aqueous system. The phosphoric acid is added to the lime as quickly as possible. The particle sizes were between -20 and +40 (425 microns) U.S. Standard Mesh. Smaller particles were not disclosed.
U.S. Pat. No. 4,292,294 disclosed a process to produce submicron-sized particles of spinel of iron and other metals involving reacting an aqueous solution of ferrous sulfate and a metal nitrate with a basic solution at room temperature and subsequent exposure to elemental oxygen.